Conventional ultrasound scanners create two-dimensional B-mode images of tissue in which the brightness of a pixel is based on the intensity of the echo return. Alternatively, in a color flow imaging mode, the movement of fluid (e.g., blood) or tissue can be imaged. Measurement of blood flow in the heart and vessels using the Doppler effect is well known. The frequency shift of backscattered ultrasound waves may be used to measure the velocity of the backscatterers from tissue or blood. The Doppler shift may be displayed using different colors to represent speed and direction of flow. In power Doppler imaging, the power contained in the returned Doppler signal is displayed.
Conventional ultrasound transducers transmit a broadband signal centered at a fundamental frequency .function..sub.0, which is applied separately to each transducer element making up the transmit aperture by a respective pulser. The pulsers are activated with time delays that produce the desired focusing of the transmit beam at a particular transmit focal position. As the transmit beam propagates through tissue, echoes are created when the ultrasound wave is scattered or reflected off of the boundaries between regions of different density. The transducer array is used to transduce these ultrasound echoes into electrical signals, which are processed to produce an image of the tissue. These ultrasound images are formed from a combination of fundamental (linear) and harmonic (nonlinear) signal components, the latter of which are generated in nonlinear media such as tissue or a blood stream containing contrast agents. With scattering of linear signals, the received signal is a time-shifted, amplitude-scaled version of the transmitted signal. This is not true for acoustic media which scatter nonlinear ultrasound waves.
The echoes from a high-level signal transmission will contain both linear and nonlinear signal components. There are a number of classical acoustic phenomena that are either non-reciprocal or nonlinear. In the non-reciprocal category are phenomena such as: mode conversions from compression waves to shear waves, mode conversion to harmonic motions, total internal reflection along boundaries (exceeding Bruster's angle), and simple refractive bending of ray paths, plus other similar phenomena. In the nonlinear category are a great many things. The generation of second harmonics by free bubbles is well documented, as indicated above. Many components of the media should become nonlinear as the wave intensity is increased to the point that the molecular "spring constants" begin to exceed their linear or small signal range. Any nonlinearity in the media should cause a very rich form of harmonic generation, and not just the second harmonic variety. In the theory of nonlinear circuits, the signals are expressed as an infinite sum of signal components. The first term in this sum is the linear term and the higher-order terms represent signals whose spectra are multiple convolutions (in frequency) of the original spectrum. This means that much more spectral energy than only the second harmonic is created. These extra spectral components can be all over the original band as well as out of the band.
In certain instances ultrasound images may be improved by suppressing the fundamental and emphasizing the harmonic (nonlinear) signal components. If the transmitted center frequency is at f.sub.0, then tissue/contrast nonlinearities will generate harmonics at .function..sub.0 and subharmonics at .function..sub.0 /k, where k is an integer greater than or equal to 2. [The term "(sub)harmonic" will be used herein to refer to harmonic and/or sub-harmonic signal components.] Imaging of harmonic signals has been performed by transmitting a narrow-band signal at frequency .function..sub.0 and receiving at a band centered at frequency 2.function..sub.0 (second harmonic) followed by receive signal processing.
A nonlinear imaging system using phase inversion subtraction is disclosed in U.S. Pat. No. 5,632,277 to Chapman et al. First and second ultrasound pulses are transmitted into the specimen being imaged in sequence and the resulting receive signals are summed. The first and second pulses differ in phase by 180.degree.. If the ultrasound waves undergo nonlinear propagation or nonlinear interaction with contrast agents or other nonlinear scattering media, then the returned signal will have both linear and nonlinear components. Upon summation, the linear components will cancel, leaving only the nonlinear components to be imaged. U.S. Pat. No. 5,706,819 presents an almost identical method for canceling the linear term while combining the nonlinear terms. In both patents the context of the invention is to take a single short "pulse" and alter its polarity from the first transmission to the second transmission. The two subsequent echoes are added to form an image of the nonlinear media. Applying the terminology used herein, these prior art operations may be interpreted as applying an N=1 code (where N represents the length of the code) to a single broadband "pulse". Neither patent considers the approach of creating an extended transmission by convolving wavelets (having certain desirable spectral properties) with a code, and designing that code to simultaneously affect the decoded linear term and the (sub)harmonic term.